The photoconductive semiconductor switch device PCSS converts an optical signal into an electrical signal. In particular, the PCSS may convert a pulse-shaped optical signal in very high frequency or terahertz frequency bands into an electrical signal to generate an electromagnetic wave.
The photoconductive semiconductor switch device operates as follows. First, photons incident from the outside thereto are absorbed by a semiconductor layer therein to generate electron/hole pairs therein. The electron/hole pairs are separated and then accelerated by an electric field generated in the semiconductor layer by a voltage applied thereto from the outside, thereby to have a high kinetic energy. Thus, avalanche multiplication of the carriers may occur due to the high kinetic energy. The separated electrons and holes are collected by two electrodes of the switch device respectively. In this way, the external light is converted into an electrical signal while generating the electromagnetic wave. In order for the photoconductive semiconductor switch device to form a very short electrical pulse via fast electrical response, the mobility of the electrons and holes in the semiconductor layer must be higher, and the carrier lifetime must be short. The mobility of the carrier defines a rising time of the pulse and the lifetime of the carrier defines a falling time of the pulse.
On the other hand, in order to the photoconductive semiconductor switch device to switch the high power, the photoconductive semiconductor switch device must prevent the current from flowing by generating a high resistance in the absence of the incident light thereto, whereas, in the case of the presence of the incident light thereto, the switch device should sufficiently reduce the resistance in an entire path of the current resulting from the flow of the electrons and holes in the semiconductor layer. Briefly, the difference between the resistance in the presence of the incident light and the resistance in the absence of the incident light is one of the most important performance indicators of the photoconductive semiconductor switch device operating at the high voltage and high power. Specifically, in order that the current in the switch device does not flow even when a voltage of hundreds to thousands KV is applied to the photoconductive semiconductor switch device, thereby to maintain a circuit enabled by the switch device in an open state, the resistance of the photoconductive semiconductor switch device in the absence of the incident light thereto should be Giga Ω or higher to suppress the current to a very low level. As a result, it is desirable that the semiconductor layer of the photoconductive semiconductor switch device acts as an insulator in the absence of the incident light.
Further, a maximum allowable voltage for the photoconductive semiconductor switch device may be influenced by a maximum allowable electric field strength in the semiconductor layer, and a distance between the two electrodes provided in the photoconductive semiconductor switch device. However, the maximum allowable voltage for the photoconductive semiconductor switch device may be actually more influenced by deteriorations thereof due to a current filament generated around each of the electrodes, the semiconductor layer breakdown, and deteriorations thereof due to a flashover on the semiconductor layer surface when the high current flows in the photoconductive semiconductor switch device. Due to these constraints, the operating voltage of the photoconductive semiconductor switch device may be limited to be a voltage much lower than a theoretical dielectric breakdown voltage. In other words, the maximum allowable voltage for the photoconductive semiconductor switch device is limited by the deteriorations thereof due to the breakdown and/or flashover around the electrode and/or on the semiconductor surface.
In order to manufacture the photoconductive semiconductor switch device capable of withstanding the high voltage and high output, a structure or a process for the PCSS capable of suppressing the flashover on the electrode surface and the current filament around the electrode is necessarily required.
FIG. 5 is a cross-sectional view of a photoconductive semiconductor switch device according to a prior art, wherein one electrode region is enlarged. Referring to FIG. 5, since the electrode prevents the incident light from being incident on the semiconductor, the resistances of the semiconductor regions nearby the edge of the electrode and beneath the electrode are very large. Therefore, the switch device deterioration phenomenon mainly starts from the semiconductor regions nearby the edge of the electrode and beneath the electrode.
Specifically, when operating the photoconductive semiconductor switch device at a high voltage, a very high electric field generated in the semiconductor layer energizes a small amount of electrons and holes generated by the absorbed photons thereto. Further, the accelerated electrons and holes are exponentially increased in number via the avalanche multiplication as described above. The photoconductive semiconductor switch device operating in the voltage range with such an optical gain may be called a non-linear photoconductive semiconductor switch device.
At this time, a very high current flows via the current filament generated in the semiconductor layer. When the high current flows in the semiconductor layer, the significant electrical energy is consumed in the semiconductor region with a higher resistance, and, thus, the temperature rises due to an ohmic loss in the semiconductor region with the higher resistance. As a result, a defect may be generated in the semiconductor region, and a metal or a semiconductor may be burned around the electrode. This becomes a major factor limiting the operating voltage of the photoconductive semiconductor switch device.